Steering system suppliers have used either a relative hand wheel angle (HWA) sensor or an absolute HWA sensor, or have not used a HWA sensor. The factors in deciding which HWA sensor to use or not to use a HWA position sensor are related to cost vs. performance/feature requirements of a given steering system.
An absolute position sensor usually is a multi-turn device, as a number of turns that the steering hand wheel makes from stop to stop is typically between three and five. In some cases, in order to have a reliable HWA measurement, a physical sensor is required. If a HWA measurement is required at key on, an absolute sensor is required. If the HWA measurement is not required until sometime after the key on and after some driving, an algorithm could learn where the center position of the hand wheel is and provide a calculated absolute position after some learning. In the latter case, the HWA could be measured by a relative hand wheel position sensor at the hand wheel or by a motor position sensor. In both cases, a physical sensor is required to monitor the hand wheel position. Physical sensors cost in building steering systems.